We have developed a new tool for guiding invasive procedures with ultrasound, based on a very simple idea. We attach a half-silvered mirror and a small flat-panel monitor to an ultrasound transducer. This resulting device, which we call the sonic flashlight, can project a virtual image of an ultrasound scan into its proper visual location within the patient, without requiring any tracking or head-mounted apparatus. The sonic flashlight provides an intuitive merger of the visual exterior of the patient with the ultrasound image in situ, in vivo, and in real time. It permits the operator to guide a needle through the skin aiming directly at the ultrasound image, using natural hand-eye coordination rather than looking away from the patient at a screen. We believe it will increase accuracy, safety, and speed for a wide variety of diagnostic and invasive procedures, and enable them to be performed with less training. Our ultimate goal is to bring the sonic flashlight to clinical practice. Towards this end we propose to improve the present design in significant ways and to test the sonic flashlight on phantoms and animals, establishing the limits of accuracy and gaining a greater understanding of the human skills involved. Specific Aims: (1) Improve the design of the present 2D sonic flashlight, including its size, weight, ultrasound quality, Doppler capability, mirror optics, ergonomics, etc. (2) Develop a matrix-array 3D sonic flashlight, incorporating a Real-Time 3D (RT3D) ultrasound scanner that will permit the sonic flashlight to display real-time in situ slices with any orientation relative to the transducer. (3)Test human performance using the sonic flashlight on specially developed phantoms, with physical exteriors and simulated ultrasound interiors containing a variety of targets. A mock-up of the sonic flashlight and a needle will be tracked to test human performance in terms of specific psychological processes. (4) Validate feasibility and repeatability of specific image-guided procedures. Then perform statistical study comparing performance of the sonic flashlight to conventional methods. Clinical consultants will be involved at every stage, from studying and redesigning the sonic flashlight to conducting animal trials. If successful, at the completion of the proposed four years of research the sonic flashlight will be ready for testing in humans to guide invasive procedures.